People have long been accustomed to making telephone calls using a handset whose earpiece portion the user typically presses up against her ear (in order to better hear the voice of the other party.) As mobile phones became prevalent, the acoustic system that delivers sound to the user's ear was faced with a variety of environmental conditions, in particular varying levels of background noise. A mobile phone might be used in a quiet room until the user walks outside to a noisy street. The intelligibility of sound from the earpiece (produced by the so-called receiver or earpiece loud speaker) is reduced when there is a raised ambient noise level. In addition, it is typical for the user in that case to want to press the handset more firmly to his ear. If the receiver is too loud, the user may pull back or lift off the handset to move the earpiece away from her ear.
The earpiece has been conventionally designed in such a way that it produces good sound pressure and quality, when it is mostly sealed or in a particular sealing condition against the user's ear. If there is a larger gap, that is a larger leak between the earpiece and the user's ear, this usually causes a significant weakening of the sensed sound pressure. Manufacturers try to ensure that the volume and frequency distribution of the sound from the earpiece is in accordance with their specification, in actual or real operating conditions where the earpiece is rarely completely sealed against the user's ear. In other words, the handset has to have the ability to tolerate or adapt to such acoustic leakage in its earpiece region.
Typically, several techniques have been used to improve acoustic leak tolerance. In one technique, a loose coupling is arranged to the receiver (which produces the sound waves in the earpiece region). The receiver in that case is acoustically loaded by a relatively large volume that is as large as possible. Another way to improve leak tolerance may be to lower the acoustic output impedance of the earpiece arrangement by using an acoustic return path.
In another solution, an acoustic sensor is added to measure the sound pressure that is present in the acoustic interspace between the earpiece region and the user's ear. A control circuit compensates for the measured loss in sound pressure that is caused by an acoustic leak, to keep the subjective impression of the loudness level (and thus the intelligibility of the speech signal), always approximately in the same range. The subjective impression of the loudness level depends not only on the total power of the acoustic signal, but also on the distribution of the energy within its signal spectrum.